The invention relates to an apparatus and a method for decomposing pollutants in waste gases of combustion processes.
These include, for example, poisonous pollutants, such as NOx in waste gases from combustion processes, particularly in exhaust gases of vehicles or stationary engines and in flue gases of power plants, operated with fossil fuels.
A different area of use of the invention is the decomposition of soot-containing pollutants in waste gases of combustion process, particularly in exhaust gases of diesel vehicles or of stationary engines.
Aside from the traditional method for eliminating poisonous and soot-containing pollutants from combustion processes, it has also already been proposed that these waste gases be treated in a dielectrically hindered discharge.
The phenomenon of the dielectrically hindered discharge, frequently also referred to as silent discharge or alternating voltage discharge between insulated electrodes has long been known. This form of discharge is distinguished owing to the fact that it can be carried out at normal pressure and at overpressures. The pressure range for carrying out such discharges extends from a few 10 mbar up to a few bar and tie distance between the electrodes lies between 0.1 mm and a few mm. Dielectrically hindered discharges are characterized owing to the fact that at least one dielectric is disposed between the electrodes or on one of the electrode. The discharge is conducted with alternating voltages ranging from a few Hz to several 100 kHz. In the case of electrodes having a large surface area, numerous, randomly distributed, small discharge threads, also referred to as filaments, are formed. Due to the insulation, the discharge after the breakdown is limited automatically and the duration of the discharge generally amounts only to fractions of the duration of the half period. As a result, the heating of the gas by the plasma is not worth mentioning.
It is furthermore known that chemical compounds can be produced or destroyed with such plasmas. Contributions to this subject are contained, for example in: xe2x80x9cProceedings of the NATO Advanced Research Workshop on Non-Thermal Techniques for Pollution Controlxe2x80x9d, Cambridge, September 1992, published by B. Penetrante and S. Schultheis, xe2x80x9cNon-Thermal Plasma Techniques for Pollution Controlxe2x80x9d, Springer Verlag Berlin 1993.
In technical solutions, the dieletrically hindered discharge is part of a plasma reactor. As a rule, the reactor is a large capacity structure and, since the electrode area of the dielectrically hindered discharge can be scaled at will, a structure with a large surface area, so that large flows can also be treated. The shaping is either planar or coaxial. A corresponding apparatus is described, for example, in the Gennan Offenlegungsschrift 37 08 508.
It has also been proposed, for example, in the German Offenlegungsschrift 195 25 754 A1 and the German Offenlegungsschrift 195 25 749 A1 to divide the reactor volume into spatially periodic structures, so that, in the flow direction, discharge zones and discharge-free zones are formed. At the same time, in the region of the discharge zones, the shaping has means for raising the field. Furthermore, provisions are made in the Germnan Offenlegungsschrift 195 25 749 A1, to introduce chemically effective materials in the region of the surfaces of the structures.
In the German Offenlegungsscrift 195 34 950 A1, a reactor is described, which consists of several modules with a plurality of parallel channels, spatially separated from one another in a dielectric body, in which electrodes are mounted.
A further version for the construction of a dielectrically hindered discharge has been proposed in the German patent 43 02 465 C1. For this, at least one electrode consists of a voltage-stimulated plasma.
A different possible reactor construction is disclosed in the U.S. Pat. No. 4,954,320. The apparatus contains metallic electrodes, between which a loose accumulation of dielectric insulating bodies, such as ceramic spheres, are accommodated. The apparatus of the German Offenlegungsschrift 44 16 676 A1 represents a similar variation, for which the space between the plate-shaped electrodes is filled with insulators, the whole cross section of which is traversed by channels or contains pores.
In order to influence the course of the chemical reactions in the plasma, it has also been proposed that certain additives be supplied to the waste gas that is to be treated. For example, in the German 42 31 581 A1, an inlet for admixtures is assigned to an apparatus for purifying waste gases.
In the state of the art, the stream of waste gases flows through the discharge space along the electrode surfaces, which extend parallel to one another. It enters at one end of the discharge space, formed by the two electrodes, and emerges at the other end, and does so also independently of whether or not an accumulation of insulators has been introduced between the electrodes. The residence time of the waste gas in the plasma treatment space corresponds to the controlled or uncontrolled amount flowing and to the flow velocity, which arises from the cross-sectional area of the treatment space. For physical reasons, the distance between the electrodes can be increased only to a limited extent. The flow velocity for an optimum treatment can therefore be decreased only, if the transverse extent of the discharge space can be increased greatly or if there is a plurality of parallel connections of such discharge spaces, which results in a large overall volume. This furthermore brings about an increased energy consumption as well as a decrease in the effectiveness of the purification process with respect to course of the chemical reactions. In addition, further reactions can thus be initiated, as a result of which other harmful substances or undesirable by-products are formed. Furthermore, in equipment constructed according to the state of the art, the treatment of waste gases containing soot must be continued, until the soot particles have been converted completely to CO2 and CO or are otherwise bound. If the load, the flow velocity or the amount of soot in the waste gas changes, the length must be such that decomposition is complete even in the most unfavorable case. Under practical conditions, an inappropriately long treatment space would be required and, moreover, the effectiveness of the purification process for other polluting components would be decreased due to the unnecessarily long duration of the treatment. Furthermore, the energy consumption would also be increased as a result.
It is therefore an object of the invention to provide an apparatus and a method for the apparatus, by means of which the decomposition of pollutants, especially of Nox and soot, from waste gases is improved. At the same time, economical and safe operation should be made possible in the case of all operationally related fluctuations in the soot content in the waste gas that is to be treated.
The porous electrode is arranged along the direction of flow. According to the invention, the waste gas is passed into a treatment space, which is constructed according to the principle of the dielectrically hindered discharge, at least one of the electrodes, forming this treatment space, being porous and electrically conductive and the waste gas flowing through his porous electrode. The porous electrode is configured so that it is permeable to the gaseous components, but acts as a filter for the soot particles, restraining these.
It has proven to be advantageous for the apparatus of the present invention that, with the passing of the waste gas stream through the porous electrode surface, there is a quieting of the gas stream, since the electrode surface is always larger than the cross-sectional area of the discharge space or the cross-sectional area of the flow conduit, so that the flow velocity in the region of the porous electrode is drastically decreased and an effective treatment can thus take place. Furthermore, a compact construction of the apparatus becomes possible and the use of energy is decreased. It is furthermore advantageous that, independently of the different soot contents of the waste gas, emission of the soot to the surrounding atmosphere is avoided. Furthermore, due to the decomposition of soot in the plasma, neither a high temperature of the waste gas nor a device for heating is required, in order to cause the carbon to react. As a further advantage, it has been observed that constituents, set free by the decomposition of poisonous components of the waste gas, such as Nox is bound by the carbon and an autoxidation of other, undesirable intermediate products is limited in this way. Through the use of a porous electrode in the treatment space, it is furthermore achieved that other harmful by-products of the waste gas, especially in the form of aerosols, are largely retained and decomposed.
For the associated method, the waste gas stream, which is to be treated, is passed into one of the spaces constructed pursuant to the invention and a plasma treatment of the waste gas takes place before the latter flows through the electrode wall into one or several adjacent spaces. The adjacent space, in turn, can be also be designed as a configuration for operating a dielectrically hindered discharge, in which a further decomposition of the intermediate products, formed in a first treatment space, takes place. The treatment optionally may be continued in further, adjacent spaces. It is advantageous that, in this way, a treatment of the waste gases can take place stepwise and complex reactions of the starting materials with the decomposition products are thus limited in this manner. In this way, the porous electrode can also be used once or several times for quieting the gas stream.
The invention is to be described in greater detail by means of examples. Examples 1a to 3 relate to an inventive apparatus and to the method of decomposing poisonous pollutants in waste gases. Examples 4 to 6 show an apparatus and the method for decomposing soot-containing pollutants in waste gases of combustion. In the drawings,